This invention relates in general to mechanical compression packing for controlling leakage about shafts, and in particular to packing which does not undergo undesired deformation when placed under load in normal use.
Because there is a wide variety of applications for mechanical packing and seals, including packing for pumps, valves, hydraulic, and pneumatic equipment, a whole industry has grown up in their design and construction. In the areas with which the present invention is concerned, the packing is generally sold in relatively long coils of braided packing material of square or rectangular cross-section from which many suitable lengths may be cut. Conventionally, several lengths are cut from the coil of material for a given installation, each length being formed into a ring about a shaft with the cut ends abutting each other. Often, as many as a half-dozen such rings are disposed about the shaft with their radial sides in abutting relationship.
A so-called packing or stuffing box formed integrally with, and generally extending outwardly from, the housing surrounds the shaft. The interior of the stuffing box is of a diameter sufficiently greater than that of the shaft to accommodate the packing rings. An annular gland is fitted about the shaft and bolted to the exterior of the stuffing box in such a fashion that an end of the gland compresses the packing rings in the stuffing box. Generally, the gland has a flange through which bolts pass which are threaded into the stuffing box. Tightening of the bolts pulls the gland toward the housing and compresses the packing rings within the stuffing box. Under such compression, the materials tend to expand radially to some extent and substantially fill the stuffing box to prevent or minimize the escape of the contents of the housing at the intersection of the shaft and the housing.
In the original manufacture of the packing material, it is braided in the form of a relatively straight length. Because it is then cut to desired short lengths each of which is formed into a ring about a circular shaft, the outside circumference of each packing ring is longer than the inside circumference. Before compression from the gland is applied, the cross-section of each ring tends to form itself into a trapezoidal shape, the narrow side of the trapezoid being the stretched side adjacent the inner surface of the stuffing box. Conversely, the wide side of the trapezoid abuts the shaft. Because of the trapezoidal cross-section which is assumed by each packing ring, the phenomenon is known as "keystoning".
Several problems arise from this keystoning effect. First and foremost, when a plurality of abutting rings are used in a typical application and the packing is compressed by the gland, force is concentrated on the packing ring corners. Severe wear of the shaft under the packing ring corners and of the packing itself at the inner corners is then encountered.
Moreover, the rapid wear of the packing rings foreshortens the useable life of the packing and shaft and reduces the time before leakage becomes intolerable, necessitating adjustment of gland compression.
Various alternatives have been proposed to combat the unwanted effects of keystoning. One alternative involves the use of packing rings which are interspersed with compensating rings machined or die-formed into wedge shapes having the wider axial dimension at the outer diameter. The packing rings are then installed in proper sequence with the "wedge spreaders" to compensate for keystoning. This expedient is useful and has provided some relief from sealing problems, but it is expensive and requires special forms of packing rings and wedges which must be carefully assembled and installed in the proper order.
It is therefore a primary object of the present invention to alleviate problems caused by keystoning of packing rings.
Another object of the present invention is to avoid the use of packing rings and wedges of preshaped cross-sections which must be assembled in a particular sequence.
Still another object of the invention is to avoid the need to die-form packing rings so that they are pre-shaped to fit the stuffing box.
A further object of the invention is the production of mechanical compression packing which is not subject to keystoning.